The present invention relates to a shape measurement method and its system for measuring the three-dimensional shape of a pattern of a semiconductor device.
As described in Semiconductor Equipment Association of Japan (SEAJ) “2005 Semiconductor Manufacturing Technology Road Map Reports No. 5 Measurement” (Non Patent Literature 1), a length measuring SEM for measuring critical dimension (CD-SEM), a scanning electron microscope (hereinbelow abbreviated to “SEM”) specialized for semiconductor measurement, is most widely used as a pattern dimension management tool in semiconductor process. FIG. 22A shows the principle of a length measuring SEM 2200. An electron beam 2202 emitted from an electron gun 2201 is narrowed with a converging lens 2203, passed through an objective lens 2205 and two-dimensionally scanned on a sample 2206 with a polarizer 2204. Secondary electrons 2207 generated from the sample 2206 with the electron beam irradiation are captured with a detector 2208, and processed inside an image processing unit 2209, thus an electron beam image is obtained. The obtained electron beam image is displayed on a display 2211 of an output unit 2210. Since more secondary electrons 2207 are generated at a pattern edge, the signal level of a part of the electron beam corresponding to the pattern edge is high as indicated with its output waveform signal 2220 in FIG. 22B, and the part corresponding to the pattern edge becomes a bright image. In the length measuring SEM, dimension measurement is performed by obtaining an inter-edge distance 1 as shown in FIG. 22B. Further, the electron gun 2201, the converging lens 2203, the polarizer 2204, the objective lens 2205 and the like are controlled with a control unit 2212.
Various methods have been proposed as a dimension measurement method, however, (a) threshold method and (b) model based measurement method will be described here.
The threshold method is disclosed in Japanese Patent Application Laid-Open Publication No. Sho 55-72807 (Patent Literature 1). In the threshold method, as denoted by numeral 2220 in FIG. 22B, when peak parts with large signal amount corresponding to left and right pattern edges are respectively referred to as a left white band (left WB) and a right white band (right WB), a Max value and a Min value are obtained in the respective left and right WBs, and a threshold level to internally divide these values at a predetermined ratio th (%) is calculated, and a cross position between the threshold and the signal waveform is defined as an edge position.
FIGS. 4A and 4B show the relation between cross-sectional shapes 411, 421 and SEM signal waveforms 412, 422. In the threshold method, as shown in FIGS. 4A and 4B, as the signal waveforms 412 and 422 change in accordance with the cross-sectional shapes 411 and 421, even when the threshold levels are equal, the relations between the detected edge positions and edge positions of the measurement subject pattern are not always equal. For example, in FIG. 4A, an edge point of a tapered edge 413 detected with a threshold level of 50% is inner by 0.5 nm from a bottom edge point, however, that of a vertical edge 423 in FIG. 4B is outer by 2.5 nm. In this manner, the dimension obtained by the threshold method is an inter-edge distance representative value to the end, and it is impossible to know the height to which the measured dimension corresponds.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2009-198339 (Patent Literature 2) discloses, regarding a pattern measuring method based on SEM image utilizing electron beam simulation, high precision pattern measurement by using a simulation image with appropriately-set shape and dimension, having much influence to the accuracy of matching measurement between simulation and real image.
In recent years, strict pattern dimensional management is required in accordance with miniaturization of pattern. There is an increasing need for measurement of subject pattern cross-sectional shape, more particularly, a dimension at a predetermined height (a bottom dimension, a middle dimension, a top dimension or the like) in place of the dimension representative value as described above.
The model based measurement method has been made to respond to this need. FIG. 3 is a principle diagram of the disclosure in J. S. Villarrubia, A. E. Vladar, J. R. Lowney, and M. T. Postek, “Scanning electron microscope analog of scatterometry,” Proc. of the SPIE, Vol. 4689, pp. 304-312 (2002) (Non Patent Literature) 2. As shown in FIG. 3, the pattern cross-sectional shape is represented with plural parameters (hereinbelow, “shape parameters”. In FIG. 3, a side wall inclination angle and top roundness are shape parameters). SEM signal waveforms of various cross-sectional shapes are obtained by simulation, and a library is created in advance. Upon dimension measurement, the edge shape and edge position of a subject pattern are estimated by selection of a waveform best corresponding with a real waveform in the library and positional shift of the library waveform. When the calculation of SEM signal waveform creation process in the simulation is appropriate and the shape parameters are appropriate, in principle, it is possible to obtain the cross-sectional shape of the subject pattern by the model based measurement method.